This invention relates to the field of lithography and more particularly to novel photopolymers useful as light-sensitive coatings for lithographic plates, to novel monomers, homopolymers, and copolymers and to novel methods for preparing and using said monomers and polymers.
In the art of lithography, the instrument used for printing is an exposed and developed plate constituted by a hydrophilic oleophobic substrate covered in the image areas by an oleophilic hydrophobic coating. Typically, the substrate is a thin sheet of metal, such as aluminum, magnesium or zinc, and the coating corresponding to the image area to be produced consists of a water-insoluble material, for example, a diazo or azide compound. In printing the desired image on a surface, the plate is first contacted with a water solution which is repelled by the image areas, but retained by the non-printing areas. Then the plate is contacted with an oil-base ink which spreads uniformly over the image area but is repelled by the non-image areas of the substrate which have retained the water solution. The ink-laden plate is then pressed against the printing surface to produce the desired image on that surface.
To prepare a printing plate of the character described, a coating of a soluble light-sensitive material is applied uniformly over the surface of the substrate. Light is then projected through a transparent photograph (normally a negative) of the image onto the plate. In those areas where light passes through a negative and strikes the light-sensitive material, the latter is chemically converted into a hard water-insoluble oleophilic material. The areas of the coating unaffected by light retain the same chemical character that they originally possessed. A developer or solvent, such as water, an alkaline solution, gum arabic, or an organic solvent is then applied to the surface of the plate to dissolve and remove those portions of the coating which have not been subjected to light, leaving unaffected the image areas of the coating which have been converted by light into an insoluble material. The oleophilic layer remaining on the plate after treatment with the solvent thus assumes the configuration of the image to be printed. Positive working light-sensitive materials are also available. Such materials are initially insoluble in the developing solution but are converted to a soluble material where they are struck by light, and a developer is employed to dissolve the soluble material from the light-exposed areas. Exposure of plates coated with such materials is therefore effected by projection of light through a positive rather than a negative.
There are numerous light-sensitive resins or materials that can be used in preparing lithographic plates, and numerous processes by which such plates are produced. One process which provides a high quality plate is the so-called "Deep Etch" process wherein the plate is chemically etched in the exposed areas. However, the Deep Etch process is complex and expensive and the use of non-etched negative working plates largely predominates in this country. The light-sensitive materials which have found most common use in this country, until recently at least, are the so-called diazo resins, such as the condensation product of paraformaldehyde with the sulfate salt of paradiazodiphenyl amine (prepared as described in U.S. Pat. No. 2,714,066). Diazo type light-sensitive coatings for lithographic plates have proved satisfactory in many respects, but are rather fragile nd must be reinforced by developing lacquers in order to withstand the wear and tear of printing. Diazo coatings also suffer from the disadvantage of being subject to fairly rapid deterioration on storage after application to the surface of a plate, particularly on storage of the plate at elevated temperatures. Such deterioration results in part from reaction of the diazo material with the underlying metal substrate. Aluminum substrates, which in most other respects represent the preferred substrate material, present a particular problem with respect to deterioration of diazo type lightsensitive materials.
To avoid the problems associated with the use of diazo resins, efforts have been devoted in the art to the provision of base plates having barrier coatings designed to prevent reaction between the resin and the metal substrate, while other efforts have been devoted to the development of various photopolymers which are relatively unreactive with the substrate. Typical of the barrier layers which have been developed are those disclosed in U.S. Pat. Nos. 2,714,066, 3,020,210, 3,064,562, 3,136,636, 3,136,639, and 3,148,984. A substantial amount of research in the art has been allocated to the development of photopolymers. Illustrative patents which describe certain previously known photopolymers include 2,610,120, 2,691,584, 2,725,372, 2,751,296, and 2,835,656. The basic objective of most photopolymer research activity has been the provision of linear polymers soluble in a variety of solvents and having pendant groups which crosslink on exposure to light to produce a hard insoluble polymeric matrix.
Prominent among the efforts in this direction has been the development of the various polymers derived from vinyl cinnamate. Ideally, vinyl cinnamate can be polymerized through the vinyl group to produce a linear photopolymer having pendant cinnamate groups. On exposure to light, the cinnamate groups should be photo-crosslinkable to produce a hard insoluble substance which would serve as a printing surface for lithographic plates. Unfortunately, however, vinyl cinnamate suffers from certain serious drawbacks. Because of the relative proximity of the double bond of the vinyl group to the double bond of the cinnamate group, vinyl cinnamate suffers from an inordinate tendency to lactonize during attempts to polymerize it. Lactonization produces a product which is not light sensitive. Even if lactonization is avoided, however, polyvinyl cinnamate polymers have not proved to be fully satisfactory in use. Exposed polyvinyl cinnamate plates are relatively fragile and cannot be rub developed. They must be spray developed, which often results in incomplete removal of the unexposed polymer and consequent scumming during a printing run.
While certain other photopolymers developed heretofore have been reasonably satisfactory, they have not generally been capable of yielding a lithographic printing surface whose abrasion resistance is as great as might be desired. Certain of the other previously known photopolymers have also presented processing problems in their preparation and application to the surface of a lithographic plate. An unfulfilled need has existed in the art, therefore, for novel photopolymers which avoid these problems, particularly as regards susceptibility to abrasion.